In a treatment of sewage and waste water, it is known that the adsorbing activity of active carbon is effective for removal of organic matter. The adsorptivity of the carbon may be expressed in terms of reduction of BOD (biological oxygen demand) and COD (chemical oxygen demand), as well as reduction of color and removal of mold smells, alga smells and other unpleasant smalls generated in eutrophic lakes and marshes.
Columns packed with active carbon heretofore used for the adsorption treatment of water and waste water are roughly divided into three types; the fixed bed type, the fluidized bed type and the moving bed type. In the case of the fixed bed type column, granular active carbon is contained in a vessel in the form of a layer having a thickness of 1 to 3 m, and water to be treated is passed downwardly through this layer to effect the adsorption treatment. In this method, when the used active carbon is to be regenerated, the supply of water to be treated is stopped, and all of the used active carbon is removed at one time and replaced by a batch of regenerated active carbon.
In operation by this method, jamming occurs in the active carbon layer by build-up of solids contained in small amounts in the water under treatment and passage of water becomes increasingly difficult. In this case, it is necessary to remove the solids caught in the layer, at least from the surface portion of the active carbon layer, by back-washing with water.
In the fluidized bed type column, granular active carbon is charged into a vessel, and water to be treated is passed upwardly through the vessel at a speed such that the active carbon is fluidized in the vessel. In this method, any solids in the running water pass through the fluidized layer of active carbon and hence, no jamming occurs and back-washing with water need not be performed. Further, since used active carbon is withdrawn from the bottom of the vessel and regenerated active carbon is supplied to the head of the vessel, replenishment of active carbon can be greatly facilitated. In this fluidized bed method, however, if the speed of the upward stream of water to be treated is too slow, active carbon is sedimented in the lower portion of the vessel and no fluidized bed is formed. On the other hand, if the speed of the stream is too high, the fluidized bed expands excessively, so that active carbon particles frequently fly out from the vessel and the operation becomes unstable. Accordingly, the contact time between water to be treated and active carbon cannot be freely made longer or shorter, and in this method, effective purification cannot be expected when a relatively long contact time is required, as for example, when a high content of sewage or the like is present or in the treatment for reduction of pulp waste water and the treatment for removal of COD (chemical oxygen demand).
In the moving bed type column, water to be treated is passed through a layer of active carbon contained in a vessel in the form of an upward stream, and regenerated active carbon is supplied from the head of the vessel while used active carbon is withdrawn from the bottom of the vessel. This replenishment of active carbon is performed intermittently. In this method, jamming is caused in the lower portion of the active carbon layer by solids contained in the running water and the resistance to passage of water gradually increases, resulting in stoppage of flow of water. Therefore, it is necessary to remove the solids caught in the active carbon layer by back-washing. In this case, however, the adsorption zone of active carbon is destroyed or disturbed by back-washing and hence, it becomes impossible to remove only active carbon saturated with adsorbed materials, resulting in reduction of the adsorption efficiency. Further, in this method, if the speed of the liquid is increased, the operation is made unstable because of fluidization of the active carbon layer, channelling and blow-by of the liquid and hence, the operation efficiency is drastically lowered.